Tissue culture lines established in this laboratory, and subsequently in others, from the erythroleukemic cells of mice infected with Friend leukemia virus, provide a widely used model for the study of the mechanisms regulating virus multiplication as well as erythroid differentiation. The cells can be stimulated to synthesize hemoglobin and to differentiate when grown in medium containing DMSO or one of a variety of other compounds, such as hexamethylene bisacetamide, hypoxanthine and butyric acid. Virus production is also enhanced. Comparative studies on variant clones, which are either not inducible by DMSO or inducible for either hemoglobin synthesis or virus production but not both, indicate that the mechanisms controlling virus proliferation and erythroid differentiation are separate. There does not appear to be a correlation between malignancy of the cells and their ability to produce virus. A series of compounds which either induce or inhibit the stimulation of erythroid differentiation of the erythroleukemia cells are also being studied to determine their effect on virus production.